This invention relates to improvements in the SOS technology, and more particularly to improvements in the crystalline quality of a single crystalline semiconductor layer formed on an insulating substrate.
As a semiconductor substrate for forming a semiconductor integrated circuit device (IC) therein, there has been proposed a structure in which a semiconductor single crystalline layer of Si or the like is grown on a highly insulating single crystalline substrate of sapphire, spinel or the like. Employing such structure (hereinbelow, typified by a sapphire plate on which Si is formed, and called "Silicon On Sapphire-SOS-structure"), a very thin semiconductor single crystalline layer in which semiconductor regions are to be formed can be realized without spoiling the mechanical strength of the semiconductor layer, and a high insulation can be achieved, so that a high speed IC, having high integration and low power dissipation, can be achieved.
Fundamentally, the SOS structure can be fabricated by a method similar to that of epitaxial growth of hitherto known semiconductors. With the conventional growth process, however, obtaining a satisfactory crystalline quality is difficult, for the reasons that an insulator has a very different lattice constant and coefficient of thermal expansion as compared to the lattice constant and coefficient of thermal expansions of semiconductors, and that the required thickness of the semiconductor single crystalline layer is extremely small, e.g., at most approximately 1 .mu.m. Expecially when a high-speed semiconductor device such as a logic IC is to be fabricated employing the SOS structure, the mobility of carriers within the semiconductor single crystalline layer (hereinbelow, simply termed "mobility") needs to be sufficiently increased. With the prior art SOS structure, however, it has been difficult to increase the mobility to a satisfactory level, or at least to a value comparable to that of a bulk semiconductor. Therefore, the application of the SOS structure to high-speed ICs has been limited.
The mobility in the semiconductor is lowered by all factors which cause the carriers to be scattered. The factors are, for example, the lattice structure of the semiconductor crystal, impurity atoms doped in the semiconductor, and stacking faults, dislocations and strains introduced into the semiconductor. Among them, the stacking faults, dislocations and strains are the main factors causing the carriers to be scattered in the SOS structure and have rendered it difficult to raise the mobility.
More specifically, the substrate and the semiconductor deposited thereon have different lattice constants, and they form a so-called heterojunction therebetween, so that twins, dislocations or stacking faults appear in the interface of the two in large numbers. The number of such disorders decreases gradually from the interface toward the opposite surface of the semiconductor. Since, however, the semiconductor layer is as thin as, for example, 0.5-1 .mu.m in the case of the IC of the SOS structure, a region in which the disorders have not sufficiently decreased cannot avoid being used as a part of a semiconductor element. It has therefore been impossible to make the mobility sufficiently high. By the same reason, it has been impossible to make the leakage current of the semiconductor element sufficiently low.
G. Yaron et al reported that a method of irradiating a Si layer with an excimer laser beam is effective for improving the crystalline quality of the top silicon surface of the SOS structure (G. Yaron and L. D. Hess, "LASOS-Laser Annealed Silicon On Sapphire", IEEE Trans. Electron Devices, vol. ED-27, pp. 573-578, March 1980). G. Yaron et al, aim at the crystalline quality of the top silicon surface and the reform of silicon islands. Therefore, they use the excimer laser (the wavelength .lambda. of the excimer laser is .lambda.=2,490 .ANG.). They do not refer to the crystalline quality of the bulk of the SI layer of the SOS structure.
The irradiation of a Si layer with a laser beam has also been disclosed in U.S. Pat. No. 3,585,088. According to this patent, an amorphous film of silicon deposited on a substrate is irradiated with a ruby laser beam and is turned into single crystalline silicon. The object of this patent is to turn, in general, an amorphous or polycrystalline material into a single crystalline material. This patent does not refer to the improvement of the crystalline quality of the single crystalline film.